Lawyers and intelligence experts with direct knowledge of two intercepted terrorist plots that the Obama administration says confirm the value of the NSA’s vast data-mining activities have questioned whether the surveillance sweeps played a significant role, if any, in foiling the attacks.

The defence of the controversial data collection operations, highlighted in a series of Guardian disclosures over the past week, has been led by Dianne Feinstein, chairwoman of the Senate intelligence committee, and her equivalent in the House, Mike Rogers. The two politicians have attempted to justify the NSA’s use of vast data sweeps such as Prism and Boundless Informant by pointing to the arrests and convictions of would-be New York subway bomber Najibullah Zazi in 2009 and David Headley, who is serving a 35-year prison sentence for his role in the 2008 Mumbai attacks.

Rogers told ABC’s This Week that the NSA’s bulk monitoring of phone calls and internet contacts was central to intercepting the plotters. “I can tell you, in the Zazi case in New York, it’s exactly the programme that was used,” he said. A similar point was made in anonymous briefings by administration officials to the New York Times and Reuters.